Understanding the structure of cortical synaptic circuits is key to comprehending information representation and processing in the auditory cortex. However, due to technical limitations, the general structure of cortical synaptic circuits, and how this structure determines cortical function, remains largely unknown. As a first step to addressing this issue, in this project, we will investigate the patterns of excitatory and inhibitory synaptic inputs underlying the functional responses of individual cortical neurons and reveal the synaptic mechanisms determining or shaping these response properties. In the auditory cortex, patterns of synaptic inputs can be largely reflected by their frequency-intensity tonal receptive fields (TRFs). These patterns represent basic structural properties of synaptic input circuitry underlying the functioning of individual cortical neurons. Using an in vivo whole-cell recording technique, we will determine the spectrotemporal pattern of synaptic inputs for both excitatory and inhibitory neurons in the input layers of the adult rat auditory cortex. We will dissect the thalamocortical components of excitatory inputs by pharmacologically silencing the cortex. The cell type of recorded neurons will be determined by their spiking and morphological properties. We will determine excitatory and inhibitory synaptic mechanisms for the frequency/ intensity tuning of cortical pyramidal neurons by revealing the patterns of excitatory and inhibitory synaptic inputs with in vivo whole-cell voltage-clamp recording techniques. We will explicate the contribution of thalamocortical excitaotry inputs to the response properties of cortical neurons by developing a novel pharmacological approach to effectively and specifically silence the cortex. Finally, by distinguishing cortical inhibitory neurons according to histology and physiology, we will determine response properties of cortical GABAergic interneurons, and their underlying synaptic mechanisms.